1. Field of the Invention
This invention relates to an optical code division multiplexing receiving device, and to a time gate processing method in the optical code division multiplexing receiving device.
2. Description of Related Art
In order to increase communication capacity, Optical Time Division Multiplexing (OTDM), Wavelength Division Multiplexing (WDM), Optical Code Division Multiplexing (OCDM), and other optical multiplexing technologies, in which a plurality of channels of optical pulse signals are transmitted together in a single optical fiber transmission path, are being studied. Among these optical multiplexing technologies, OCDM has the excellent characteristic that there is no limit to the number of multiplexed channels when multiplexing optical pulse signals.
Using OCDM communication, transmission is performed as follows. On the transmitting side, codes which are different for each channel are used to encode optical pulse signals. On the receiving side, decoding is performed using the same codes as those used for encoding on the transmitting side, to obtain the original optical pulse signals. Methods for encoding and decoding in OCDM communication include time-spreading methods, wavelength-hopping methods, time-spreading wavelength-hopping methods, and similar. In this invention, a time-spreading wavelength-hopping method is used.
Time-spreading wavelength-hopping methods will be explained referring to FIG. 1 through FIG. 4. For convenience of explanation, a reference to an optical pulse train refers to the entirety of optical pulses arranged at equal intervals on the time axis. Further, a reference to an optical pulse signal is used only to mean an optical pulse train which reflects binary digital electrical signals.
FIG. 1 is a block diagram of a transmission device used in OCDM communication (hereafter simply called an OCDM transmission device), which employs a time-spreading wavelength-hopping method.
The OCDM transmission device 10 comprises a multi-wavelength pulsed light source 15, optical divider 28, first through fourth transmission portions 30a to 30d, and multiplexer 50.
The multi-wavelength pulsed light source 15 is a light source which outputs an optical pulse train in which light of a plurality of wavelengths is included. Here, an example will be explained in which the multi-wavelength pulsed light source 15 outputs an optical pulse train containing four wavelength components, with the mutually different wavelengths λ1, λ2, λ3, and λ4.
The multi-wavelength pulsed light source 15 comprises first through fourth light sources 20a to 20d, an optical coupler 22, an optical pulse train generator 24, and a clock generation device 26. As the first through fourth light sources 20a to 20d, for example, semiconductor laser diodes (LDs) are used. The first through fourth light sources 20a to 20d each output continuous light at a single wavelength, the wavelengths of which are respectively λ1, λ2, λ3, λ4. The continuous light output from the first through fourth light sources 20a to 20d is input to the optical coupler 22, and is multiplexed into continuous light comprising the λ1, λ2, λ3, and λ4 wavelength components. Continuous light output from the optical coupler 22 comprising the λ1, λ2, λ3, λ4 wavelength components is input into the optical pulse train generator 24. The optical pulse train generator 24 is for example configured using an Electro-absorption Modulator (EAM). An electrical clock signal, generated by the clock generation device 26, is input to the optical pulse train generator 24. The optical pulse train generator 24 uses the input electrical clock signal to perform modulation, to generate an optical pulse train with period equal to the period of the electrical clock signal. Each of the optical pulses in the optical pulse train comprises the wavelength components λ1 through λ4.
The optical pulse train generated by the multi-wavelength pulsed light source 15 is divided into four trains by the optical divider 28, which are input to the first through fourth transmission portions 30a to 30d. The first through fourth transmission portions 30a to 30d are configured similarly, and so the first transmission portion 30a is explained as representative, and explanations of the second through fourth transmission portions 30b to 30d are omitted.
The optical pulse train input to the first transmission portion 30a is first input to the optical pulse signal generator 32. The optical pulse signal generator 32 is for example configured using an EAM. A binary digital electrical signal (in the figure, indicated by an arrow S10) is input to the gate of the optical pulse signal generator 32. The optical pulse signal generator 32 generates from the input optical pulse train and outputs optical pulse signals which reflect the signal pattern of the binary digital electrical signal S10.
The optical pulse signals output from the optical pulse signal generator 32 are input to the encoding portion 40 and encoded thereat. The encoding portion 40 is for example configured using a fiber Bragg grating (FBG), and by imparting different delays for each wavelength, encodes the input optical pulse signals. Here, the code used in encoding by the encoding portion comprised by the first transmission portion 30a is assumed to be code 11. Signals encoded in the first transmission portion 30a are called first encoded transmission signals (in the figure, indicated by an arrow S14a).
Similarly in the second through fourth transmission portions 30b to 30d, optical pulse signals reflecting the signal pattern of binary digital electrical signals are generated from an input optical pulse train in an EAM used as an optical pulse signal generator. The generated optical pulse signals are encoded using mutually different codes by the encoding portions comprised by the second through fourth transmission portions 30b to 30d. Here, the codes used in encoding by the encoding portions comprised by the second through fourth transmission portions 30b to 30d are assumed to be, for example, code 12 to code 14. As a result, second encoded transmission signals (indicated by an arrow S14b in the figure), third encoded transmission signals (indicated by an arrow S14c in the figure), and fourth encoded transmission signals (indicated by an arrow S14d in the figure) are output from the second through fourth transmission portions 30b to 30d, respectively.
The optical pulse signals encoded by the first through fourth transmission portions 30a to 30d are multiplexed in the multiplexer 50, and are transmitted. Here, signals resulting from multiplexing of encoded optical pulse signals (indicated by an arrow S20 in the figure) are here called optical code division multiplexed (OCDM) signals.
Encoding in OCDM communication using a time-spreading wavelength-hopping method will be explained, referring to FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D and FIG. 2E. Here, for simplicity, an example of dual-multiplexing OCDM will be explained, in which signals encoded using two different codes are multiplexed. FIG. 2A and FIG. 2C show optical pulses for one optical pulse signal. In the optical pulse shown in FIG. 2A, wavelength components λ1 to λ4 are included. This optical pulse is encoded using codes given by code 11. As a result, the signal is analyzed into optical pulses by wavelength components, as shown in FIG. 2B, and different delays are imparted to each wavelength component, so that the wavelength components are arranged at different positions on the time axis.
Similarly, the optical pulse signals in which light at wavelengths λ1 to λ4 is included shown in FIG. 2C are encoded using the code given by code 12, different from code 11. Through this encoding, as shown in FIG. 2D, optical pulses in which wavelength components at λ1 to λ4 are included are analyzed into optical pulses of each wavelength component, and different delays are imparted to each wavelength component, so that the wavelength components are arranged at different positions on the time axis. Signals encoded using code 11 and signals encoded using code 12 are multiplexed, to result in dual-multiplexed OCDM signals, as shown in FIG. 2E.
FIG. 3 is a block diagram of a conventional example of a receiving device for OCDM communication (hereafter simply called an OCDM receiving device), used in time-spreading wavelength-hopping methods.
The OCDM receiving device 110 comprises an optical divider 120 and first through fourth reception portions 130a to 130d. OCDM signals S20 received by the OCDM receiving device 110 are divided by the optical divider 120 and sent to the first through fourth reception portions 130a to 130d. The first through fourth reception portions 130a to 130d are configured similarly, and so the first reception portion 130a will be explained as representative, and explanations of the second through fourth reception portions 130b to 130d are omitted.
The first reception portion 130a comprises a decoding portion 140, time gate processor 150, clock extraction circuit 160, and photoelectric converter 190. The decoding portion 140 in the first reception portion 130a has the same code (code 11) as the encoding portion 40 in the first transmission portion 30a, explained referring to FIG. 1. Similarly, the decoding portions in the second through fourth reception portions 130b to 130d have the same codes (code 12 to code 14) as the encoding portions in the second through fourth transmission portions 30b to 30d, respectively.
Decoding in OCDM communications will be explained, referring to FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D. Here, decoding of dual-multiplexed OCDM signals, explained referring to FIG. 2, is used as an example. FIG. 4A through FIG. 4D is a diagram used to explain decoding of dual-multiplexed OCDM signals. FIG. 4A shows signals resulting from decoding of received OCDM signals using a code given by code 11. FIG. 4C shows signals decoded from received OCDM signals using the code given by code 12.
As shown in FIG. 4A and FIG. 4C, optical pulse signals decoded by each of the decoders comprise auto-correlated signals and cross-correlated signals. Auto-correlated signals are the signals decoded using the same code which is used for encoding. Cross-correlated signals are the signals decoded using the different code which is used for encoding. For example, in the case of optical pulse signals decoded by the decoder in the first reception portion 130a (which hereafter may be called the first decoder), optical pulse signals decoded by the first decoder from the first encoded transmission signals, obtained by encoding using the code provided by code 11, are auto-correlated signals. On the other hand, optical pulse signals decoded by the first decoder from second optical encoded signals, obtained by encoding using the code provided by code 12, are cross-correlated signals. Similarly in the case of optical pulse signals decoded by the decoder in the second reception portion 130b (which hereafter may be called the second decoder), optical pulse signals decoded by the second decoder from second optical encoded signals are auto-correlated signals, while optical pulse signals decoded by the second decoder from first optical encoded signals are cross-correlated signals.
Here, in the optical pulse signals obtained as a result of decoding, cross-correlated signals result in so-called interference noise. When the number of multiplexed signals increases due to encoding, interference noise due to cross-correlated signals increases, so that the error rate for encoded signals increases. Hence in order to reduce the signal error rate, time gate processing is performed (see for example Literature 1: Hideyuki Sotobayashi, Wataru Chujo and Ken-ichi Kitayama, “3-1 Optical Code Division Multiplexing And Its Application to Peta-bit/s Photonic Network”, Journal of the National Institute of Information and Communications Technology, Vol. 48, No. 1, 2002).
Here, time gate processing is processing to extract signals at desired time intervals. In other words, time gate processing is processing in which a gate is opened to allow decoded signals to pass only during the time in which an auto-correlated signal is input, whereas the gate is closed during the time in which a cross-correlated signal is input, so that only auto-correlated signals are extracted. FIG. 4B and FIG. 4D show signals extracted by time gate processing of signals decoded by the first decoder and by the second decoder respectively. In order to perform this time gate processing, a clock signal which specifies the time interval of the time gate is necessary. Hence in the reception portion, it is necessary to extract a clock signal of the same frequency as the electrical clock signal of the transmission portion.
In a conventional OCDM receiving device, signals decoded by the decoding portion 140 are divided into two signals, one of which is sent to the clock extraction circuit 160, and this clock extraction circuit 160 extracts the clock signal (indicated by the arrow S36 in FIG. 3) (see for example Literature 2: Hideyuki Sotobayashi, Wataru Chujo and Ken-ichi Kitayama, “Transparent Virtual Optical Code/Wavelength Path Network”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 8, No. 3, May/June 2002).
Signals which have been decoded by the decoding portion 140 are input to the time gate processor 150. As the time gate processor 150 which performs time gate processing, for example, an EAM can be used. The clock signals S36 extracted by the clock extraction circuit 160 are input to the gate of the time gate processor 150, and only the auto-correlated signals are extracted. The signals output from the time gate processor 150 are amplified by the optical amplifier 152a. As the optical amplifier 152a, for example, an erbium-doped optical fiber amplifier can be used. An ASE cutoff filter 152b is placed at the output portion of the optical amplifier 152a, and Amplified Spontaneous Emission (ASE) components contained in the signals output from the optical amplifier 152a are removed. Signals output from this ASE cutoff filter 152b are input to the photoelectric converter 190, and after conversion into electrical signals, are output.
However, in a conventional OCDM receiving device as disclosed in Literature 2, clock signals are extracted from decoded signals, and so the clock signals have substantial time jitter. Consequently, fluctuations occur in auto-correlated signals after time gate processing.
Hence upon conducting diligent research, the inventors of this application discovered that, in the reception portion, by performing extraction before decoding, that is, by extracting clock signals from encoded signals in the encoded state, time jitter can be reduced.
It is therefore an object to provide a receiving device for OCDM communication in which, by performing time gate processing using clock signals in which time jitter has been suppressed, time jitter and intensity jitter in auto-correlated signals after time gate processing, which are observed in the prior art, are reduced.
Another object of the invention is to provide a clock extraction method in a receiving device for OCDM communication.